The invention relates generally to coal drying and more specifically relates to a method of drying wet coal fines smaller than 28 mesh in size. The invention will be specifically described in connection with a method of drying the coal fines in a fluid bed type dryer heated with a coal fired burner fueled with ultrafine coal particles carried away from the fluidized bed with the exhaust gases.
In the processing of coal, it is highly desirable, and often mandatory, to remove a number of impurities from the coal prior to use. The need to remove these impurities has increased in recent years with the advent of modern mining techniques. Although these modern mining techniques are highly efficient and permit high production rates, they tend to be less discriminating than older mining techniques in the collection of material from the ground. As a result, coal mined by many of these modern techniques tends to have an even greater amount of impurities than was common several years ago. As a result, the need to process coal through a cleaning process to remove impurities has been accentuated in recent years.
There are a number of processes for removing impurities from coal. In general, these are washing type processes which leave the coal wet. It is then necessary to remove moisture from the cleaned coal through a thermal dryer or the like.
One approach to drying coal fines after an initial washing process is to pass the wet coal through a vibrating fluidized bed dryer where hot gases are forced through an orifice plate to fluidize a bed of coal to be dried on top of the plate. Other dryers in these systems use oil or gas burners to supply hot gases to the dryer, which result in poor economics, especially if the coal has high ash or sulphur content or is otherwise of low quality.
Modern mining and cleaning techniques also result in more fine material or coal fines. The coal fines have a greater surface area than larger pieces of coal and therefore have a greater capacity for retaining moisture. Not only does this fine material have a greater water retention capacity, the individual particles have a tendency to adhere to each other forming agglomerations. Consequently, the wet fines have presented considerable handling and drying problems. In the past, when coal fines were being processed, the attendant drying and handling complications usually necessitated mixing in coarser material before use of fluidized bed dryers. Users were thus relegated to using machines of much greater capacity for drying these wetted coal fines. The wetted coal fines, having a consistency very similar to wet clay and would, for example, clog up the orifices in a fluidized bed plate, disrupting the dryer's operation.
As noted above, other vibrating fluidized bed dryers in the past have used relatively expensive oil or gas heat to supply gases for drying. The reason is that coal burners inherently generate fly ash and this fly ash tends to clog up the orifices of a fluidized bed type dryer from the underside of the plate. As a result, the most economic burner for coal drying systems has not been heretofore technically feasible.